Particulate filters are able to filter soot particles out of the lean exhaust gas from internal combustion engines and thereby to prevent the particles from being emitted into the atmosphere. Various types of filters can be used for this purpose, such as what are known as wall-flow filters, ceramic fibres or foams and filters formed from wire fabrics, which allow separation rates of up to 95% and above. However, the real difficulty is not that of filtering out the soot particles, but rather that of regenerating the filters which are used. Carbon soot only burns at temperatures of approximately 600° C. However, these temperatures are generally only reached in the full-load range by modern diesel engines. Consequently, additional booster measures are required to oxidize the soot particles that have been deposited in or on the filter. A distinction is drawn between active and passive measures: in the case of active measures, the temperature of the filter is raised to above the temperature required to oxidize the soot, for example by electrical heating. Measures of this type always entail increased fuel consumption. In the case of the passive systems, the soot ignition temperature is lowered, for example by the use of organometal fuel additives, such as ferrocene, or by catalytic coating of the filter. However, this reduction in the soot ignition temperature is not generally sufficient to ensure regeneration of the filter even at low load points, and consequently nowadays a combination of active and passive measures is generally used.
The combination of an oxidation catalyst with a particulate filter has proven particularly suitable. In this case, the oxidation catalyst is arranged upstream of the particulate filter in the exhaust system. An post-injection or other measures taken at the engine causes unburnt fuel and carbon monoxide to pass to the oxidation catalyst, where they are catalytically converted into carbon dioxide and water. The exhaust gas and therefore also the particulate filter arranged downstream are heated with the aid of the heat of reaction that is released. The post-injection quantity can be reduced by using a catalytic coating, which lowers the soot ignition temperature, on the filter or fuel additives, and the filter can then be regenerated at any engine operating point.
Recently, filters close to the engine, in which what is known as a pre-catalyst is integrated on the filter substrate, i.e. the coating of the pre-catalyst is applied direct to the particulate filter substrate, have also been incorporated in passenger cars. A filter of this type close to the engine has to have a correspondingly high oxidation potential in order to be able to guarantee that the statutory emission limits for hydrocarbons and CO are adhered to over the full life cycle. Furthermore, the filter has to be able throughout its entire service life to convert hydrocarbons which are post-injected during the regeneration, in order thereby to be able to provide the exothermal energy required to burn off the soot. On account of a filter of this type being fitted close to the engine, its catalytically active coating must be thermally stable.
Hitherto, it is virtually exclusively been filter coatings comprising platinum that have been used for diesel passenger cars. In principle, coatings based on platinum and palladium have also been the subject of discussion for some time. The latter coatings are distinguished by an excellent thermal stability but have a significantly lower fresh activity compared to coatings comprising platinum alone. A further drawback of catalytic coatings containing platinum and palladium is their high sensitivity to sulphur, which has hitherto prevented them from being used for diesel engines of passenger cars. Palladium is poisoned even when exposed to diesel exhaust gas containing sulphur dioxide for a relatively short period of time, which causes a corresponding loss of activity. On the other hand, this sulphur poisoning is reversible at high exhaust gas temperatures. This means that in the case of periodically regenerating systems, such as for example applications with diesel particulate filters, during regeneration of the filter the catalyst itself is “decontaminated” at the same time.